This invention relates generally to wire transmission systems for a vortex-type flowmeter, and more particularly to a crystal-controlled, two-wire transmission system of high sensitivity, with optional linearization and totalization.
In a bluff-body type of vortex flowmeter, the vortex-shedding body is mounted within a flow conduit transversely with respect to the flow axis thereof to create fluidic oscillations whose frequency is proportional to flow rate. These fluidic oscillations are picked up by a transducer which yields an electrical signal whose frequency corresponds to the frequency of the oscillations.
In the prior Herzl U.S. Pat. No. 3,948,098, there is disclosed a vortex-shedding flowmeter in which a piezoelectric sensor actuated by vortex-pressure pulses produces an alternating voltage at a frequency corresponding to the pulse frequency. This alternating voltage is processed to produce a corresponding square-wave signal which acts, by way of an electronic switch, to control the charge/discharge action of a set of capacitors. The circuit is so arranged that the average D-C switch current is directly proportional to the frequency of the square-wave signal, and this, in turn, is directly proportional to the frequency of the sensed vortices.
A similar piezoelectric sensor arrangement is disclosed in the Richardson et al. U.S. Pat. No. 3,948,098 in which the piezoelectric signal controls the charge/discharge characteristics of a set of capacitors to produce an average charging current that reflects the frequency of vortex shedding and controls an output amplifier in a feedback arrangement to produce a varying D-C flow signal in a useful range (4 to 20 mAdc) over a two-wire transmission line leading to a remote station.
In the Richardson et al. patent, the opposing electrodes of the piezoelectric sensing element are connected through respective input resistors to the input terminals of an operational amplifier. Since piezoelectric sensors suitable for this arrangement are limited by practical considerations to very small sizes, the capacitance of such sensors is quite low--in the order of a few picofarads (pf). This dictates a very high input impedance for the associated operational amplifier, and the input resistors must therefore be of a very large value (in the megohm range).
As a consequence, the input resistors must have values in excess of one megohm each. Inasmuch as the signal output level is quite low, effective transmission of this signal becomes very difficult, for noise pick-up and stray leakage due to moisture are then difficult to avoid. With still lower operating frequencies or values of crystal capacitance, the impedance rises correspondingly to further aggravate this problem.
In the copending Herzl application Ser. No. 768,414, filed Feb. 14, 1977, there is disclosed a noise-rejecting sensing system for a vortex-type flowmeter which functions effectively within a very broad operating frequency range, the system being responsive to extremely low as well as to high frequencies. The entire disclosure of the Herzl application is incorporated herein by reference.
In the sensing system disclosed in the copending Herzl application, the sensor, which is responsive to the periodic fluidic pulses, takes the form of a resonator element such as a piezoelectric crystal or inductor that is included in the frequency-determining circuit of a relatively high-frequency oscillator. The central carrier frequency of the oscillator is determined by the normal resonance characteristics of the sensor/resonator in the absence of fluidic pulses. The fluidic pulses to which the sensor is responsive act to impose a frequency-modulation component on the oscillator carrier, this component depending on the repetition rate of the pulses which is a function of flow rate. The F-M signal is demodulated to produce an output signal whose frequency is proportional to flow rate. For purposes of transmission, this output signal is changed into an analog voltage which is fed to a voltage to-two wire converter to produce a current at a remote station in a useful range (i.e., 4 to 20mAdc).
The main advantage of a vortex flowmeter in which the sensor for the fluidic oscillations functions as the resonator of an oscillator to frequency-modulate the oscillator carrier as a function of flow rate as distinguished from a direct action sensor of the type disclosed in the Richardson et al. patent, is that it makes possible a very low impedance value. This low impedance value is highly desirable when signal transmission is required and almost completely avoids the leakage and other problems characteristic of prior arrangements.
Moreover, the resonator/sensor system disclosed in the copending Herzl application is operable over a much broader operating frequency range, for F-M works down to D-C detection levels without degradation, so that the very low operating frequencies encountered in large vortex-shedding flowmeters can be handled without difficulty.
The concern of the present invention is with the transmission of signals derived from a vortex-shedding flowmeter of the type disclosed in the prior Herzl U.S. Pat. No. 3,948,098 as well as the above-identified Herzl application wherein the obstacle assembly mounted in the flow tube, which includes a vortex-shedding body, is also provided with a rear section that is cantilevered from this body and is deflectable. This rear section is excited into vibration at a rate corresponding to the frequency of fluidic oscillations. It is these physical vibrations which are detected by the resonator/sensor to produce an F-M signal. This F-M signal is further modified by a signal conditioner to produce a corresponding 4 to 20 mAdc analog signal or to produce a corresponding digital signal which can be transmitted over a great distance to a remote station.
It is important that the relationship existing between the output signal of the flowmeter and the F-M signal be clearly understood. The frequency of the meter signal is a function of the flow rate of the fluid being metered; the higher the signal frequency, the greater the flow rate. But when an F-M carrier is modulated in accordance with fluidic oscillations, the extent to which the carrier is caused to deviate in frequency from its center frequency depends on the amplitude of the modulation component (the vortex meter signal), whereas the rate at which the deviation takes place depends on the frequency of the modulation component. Consequentially, the bandwidth of the F-M signal is effectively determined by the extent to which the rear section of the obstacle assembly is deflected by the fluidic oscillations. By means of an F-M demodulator, the F-M signal is converted into a voltage whose frequency is determined by the rate of deviation from the center frequency of the carrier and whose amplitude is determined by the extent of frequency deviation, thereby recovering the original frequency component (the meter signal).
We shall now consider how these F-M principles are applicable to a vortex flowmeter transmission system in which the modulation component is derived from the vibrating rear section of an obstacle assembly. As pointed out in Herzl U.S. Pat. No. 4,019,384, the deflectable structure is preferably relatively rigid so that the total excursion of the rear section is almost microscopic, even at the highest amplitude of fluid oscillations. In this way, metal fatigue of the supporting cantilever beam is minimized and failures are not experienced even after prolonged operation.
When, therefore, an F-M transmission system of the type disclosed in the copending Herzl application operates in conjunction with a vortex-shedding meter whose rear section is only slightly deflectable even in response to fluidic oscillations of fairly high amplitude, the resultant F-M produced by the carrier oscillator associated with the sensor/resonator exhibits a small percentage deviation. Percentage deviation is determined by the ratio between the deviation frequency times 100 divided by the carrier frequency. Hence with a carrier frequency of 1000 Hz and a deviation of 10 Hz, the percentage deviation is 1%.
Because of the small percentage deviation which occurs with a slightly deflectable rear section in a vortex-shedding flowmeter in an F-M transmission system of the type disclosed in the copending Herzl application, the F-M signal developed by the carrier oscillator is difficult to detect with a standard F-M demodulator.
The operation of a standard F-M demodulator depends on the percentage deviation of the F-M signal being detected; the greater the percentage, the better the detector response. If, therefore, one has a vortex meter with a deflectable section that undergoes a relatively large excursion even with fluidic oscillations of modest amplitude, the resultant F-M signal would exhibit a large percentage change and would present no demodulation problems.
However, in practice, the deflectable section in the interest of a rugged flowmeter design, is made only slightly deflectable, so that with the F-M system disclosed in the copending Herzl application, the percentage change in the F-M signal is inevitably quite small. For example, assuming that the carrier oscillator associated with the sensor/resonator has a center frequency of 66,000 Hz and that a particular flow rate in the meter brings about a 66 Hz deviation from the center frequency, this amounts to a mere 0.11% change and is therefore difficult to detect. One could, of course, enhance the sensitivity of the system by providing a vortex meter of greater deflectability, but this would be at the expense of an effective meter life.